Corona charging device

ABSTRACT

A negatively biased corona discharge system includes a conductive electrode having a thin inorganic dielectric outer layer bonded thereto which is employed as a corona discharge electrode. The discharge system is utilized for uniformly placing a negative charge on an insulator substrate such as an electrophotographic imaging surface. The coating on the electrode acts to suppress the widely spaced emission nodes common to all negatively biased metal corona discharge electrodes. The coated electrode may, therefore, be placed in close proximity to the substrate which it is charging and operated at low emission densities without sacrificing charge uniformity thereby reducing the power requirement of the corona discharge system.

United States Patent [191 Bingham et al.

[ Jan. 29, 1974 CORONA CHARGING DEVICE [75] Inventors: Ronald E. Bingham; Thomas F.

Cecil, both of Lexington, Ky.; Guido Galli, Saratoga, Calif.; Gereon E. Poquette, Versailles, Ky.

[73] Assignee: International Business Machines Corporation, Armonk, NY.

[22] Filed: Dec. 20, 1972 [21] Appl. No.: 317,038

[52] US. Cl. 317/2 62 A, 250/326, 313/355, 313/357 [51] Int. Cl. H0lj 1/14 [58] Field of Search 317/2 R, 2 F, 262 A; 250/495 Z; 313/354, 355, 357

[56] References Cited UNITED STATES PATENTS 3,566,l08 2/1971 Weigl et al 317/262 A 3,634,726 l/l972 Jay 317/2 F Primary Examiner-William M. Shoop, Jr. Assistant Examiner-Jimmy E. Moose, Jr. Attorney, Agent, or Firm-John Girvin, Jr.

[57] ABSTRACT A negatively biased corona discharge system includes a conductive electrode having a thin inorganic dielectric outer layer bonded thereto which is employed as a corona discharge electrode. The discharge system is utilized for uniformly placing a negative Charge on an insulator substrate such as an electrophotographic imaging surface. The coating on the electrode acts to suppress the widely spaced emission nodes common to all negatively biased metal corona discharge electrodes. The coated electrode may, therefore, be

placed in close proximity to the substrate which it is.

charging and operated at low emission densities without sacrificing charge uniformity thereby reducing the power requirement of the corona discharge system.

14 Claims, 6 Drawing Figures PAIENIED JAN 2 91974 FIG. 1

CORONA CHARGING DEVICE BRIEF BACKGROUND OF INVENTION 1. Field This invention relates to a negative corona discharge system for uniformly charging an insulating layer and, more particularly, to an improved corona discharge electrode for negatively charging an electrophotographic imaging surface.

2. Description of the Prior Art In well known electrostatic printing devices, a corona generating device including a corona discharge electrode is commonly used to place positive or negative charges onto a photoconductive surface prior to'exposing the photoconductive surface to a pattern of light. The light pattern then discharges the photoconductive surface creating an electrostatic image of the light pattern thereon which is subsequently developed with electrostatic developer material thereby making the electrostatic image visible. I

When a positive corona generated from a metallic filament electrode is used in the charging process, the resultant positve charge on the photoconductive surface is relatively uniform due to the uniformity of the positive corona electrode emission. However, when a negative corona generated from a metallic filament electrode is utilized to negatively charge the photoconduc- .tive surface, the photoconductive surface obtains a charge which varies in density frompoint to point due to the non-uniform negative corona electrode emission. This non-uniformity in charge is readily observed in the developed image since areas containing a higher charge will attract more electrostatic developer material thereto thereby creating a streaked image appearance.

Various prior art negative corona devices have been constructed in order to produce a uniform negative charge on the substrate being charged. One prior approach has been to move the metallic corona wire electrode and the surface being charged simultaneously in orthogonal directions in an attempt to average out the non-uniform charge. Such a' system is necessarily bulky and expensive.

Another prior art'approach has been toplace an alternating current in series with the high voltage direct current across the corona wire electrode. Such approaches have involved costly equipment which must be used to develop the high frequency required for good uniformity. Another prior art approach has been to raise the potential of the negative corona wire electrode thereby causing the emission points to move closer together on the corona wire. While some improvement in uniformity is noted, the resultant charge is still non-uniform and can be noticed when the charged substrate is utilized in an electrophotographic process since the non-uniform charge produces streaked copies. Further, the higher voltages require more expensive supplies and also result in increased unwanted Ozone production.

Another method used in improving the uniformity of the charge created by a negative corona electrode is to separate the substrate being charged from the corona electrode by an adequate distance so that the groups of electrons emitted from the nodal points on the electrode have adequate distance to spread out and overlap before being deposited on the surface of the substrate.

The resulting charge is always more uniform with the separation, but there is a great loss of efficiency created by the separation distance. That is, the potential on the corona electrode must be increased dramatically as the distance increases between the corona electrode and the substrate being charged.

Another prior art approach has been to heat the corona electrode thereby achieving uniform charging of the substrate by thermionic emission. The corona electrode must be heated to a sufficiently high temperature to assure good emission. However, many of the materials which are good thermionic emitters also react easily with impurities in the atmosphere. Further, such an approach would require costly electrodes and equipment for power and thermal insulation and equipment to compensate for thermal expansion. Accordingly, such a device cannot be utilized readily or economically in a machine environment.

A still further prior approach has been to utilize an alternating electric and/or magnetic field in conjunction with the negative corona electrode. The utilization of such a field makes requisite an alternating field generator at some considerable expense. Further, while uniformity of emission is somewhat improved, there nevertheless are numbers of widely spaced sites of emission.

SUMMARY In order to overcome the above noted shortcomings of the prior art and to provide a negative DC corona discharge system which provides uniform emission from the corona discharge electrode, the corona electrode of the present invention is formed of a conductive member having a thin inorganic dielectric outer layer bonded thereto. The dielectric layer acts as a suppressor of large and widely spaced nodes thereby eliminating the noding problem common to all negatively biased metal corona electrodes. Since the negatively biased corona electrode of the present invention'has an emission pattern associated therewith much like that exhibited by a positively biased metallic corona electrode, the negative electrode may be placed in close proximity to the substrate which it is charging. The proximity of thecorona electrode to the substrate and its lower emission density reduces the power requirements for a negative corona discharge system thereby greatly increasing the system efiiciency. When such a corona system is utilized in an electrophotographic copying apparatus, the uniformity of the electron emission from the negatively biased corona electrode is sufficiently uniform so that no noticeable streaking or copy quality problems are noted.

It is, therefore, an object of this invention to provide a uniform negative corona emission from a corona electrode.

It is another object of this invention to provide a simple and economical negative DC corona source for use in an electrophotographic reproduction apparatus.

A still further object of this invention is to provide a negative uniform corona source for uniformly charging an electrophotographic plate.

The foregoing objects, features, and advantages of the invention will be apparent from the following more particular description of the preferred embodiments of the invention as illustrated in the accompanying drawings.

In the drawings:

FIG. 1 is a pictorial perspective illustration partially in section of a corona discharge electrode constructed in accordance with the present invention.

FIGS. 2a-2e are schematic illustrations of typical charging systems incorporating the corona discharge electrode of the present invention positioned to charge ,a photoconductive insulator surface.

that the outer-most portion or layer of the core be of I a conductive material. Typical conductive filament materials include metallic materials such as stainless steel,

molybdenum, tungsten, aluminum, gold, copper, and the like. Typical synthetic filament materials include cotton yarn, silk yarn, rayon yarn, and the like. If such synthetic filaments are utilized, it is necessary that they be coated with an outer conductive layer.

Any suitable inorganic dielectric material or combination of such materials may be employed as the thin outer layer 15. It is, however, necessary that the dielectn'c material exhibit a resistivity of ohm centimeters or greater. Typical inorganic dielectric materials include metal oxides such as oxides of aluminum, zinc, magnesium, titanium, barium, beryllium,'calcium, cerium, strontium, zirconium, thorium, and hafnium. Typical inorganic dielectric materials further include ceramic materials such as silicone nitride, silica, silicone, boron nitride, zirconium silicate, titanates such as lead, barium and calcium, ferrites such as zinc, aluminum and magnesium and glasses such as phosphosilicate glasses (I,,Si,() where X typically varies from 0 to 33 percent and Z typically varies from 66 percent to 100 percent), borosilicate glasses (B SiQ and metallic oxide additions thereto M,,P,Si,0,

A corona discharge electrode 11 of approximately 0.0025 inches in diameter is preferred for maximum strength and optimum corona discharge characteristics. However, smaller diameter electrodes can be used at lower voltage potentials and similarly larger diameter electrodes can be utilized at higher voltage potentials, the corona potential necessary to produce the required corona current increasing with an increase in filament diameter.

The thin outer layer may be applied to the core 13 by utilizing various techniques such as chemical vapor deposition, sputtering, plating, jet coating, dipping, and various other well known application techniques depending upon the materials selected. Additionally, the core 13 and the thin outer layer 15 may be formed from the same material, a metal, which oxidizes.

It is to be further noted that while the corona discharge electrode 11 is depicted as a filament electrode, various other well known shaped electrodes such as knife edge, wedge, or strip electrodes may be utilized in accordance with the configuration desired. When such an electrode is utilized, it is necessary that the conductive portion thereof have a thin inorganic dielectric layer bonded thereto covering the emitting radius. As with a filament electrode, the potential required to produce thenecessary corona current increases with an increase in electrode radius. I

The corona discharge electrode 1 1 may thereafter be placed in various corona discharge systems including a corona system depicted in FIG. 2a, a corotron system depicted in FIG. 2b, a scorona system depicted in FIG. 20, a scorotron system depicted in FIG. 2d, or a bird cage system depicted in FIG. 2e, and connected to a negative bias source 17 as in FIG. 2a. The corona discharge electrode so constructed will emit uniformly when biased to a proper negative voltage and when placed in proximity to a conductive element.

Referring to FIG. 2a of the drawings, a three wire corona discharge system located adjacent an electrophotographic slate is schematically depicted. The electrophotographic plate 18 comprises a conductive substrate l9 and an insulating layer 21, A suitable photoconductive insulator material is disclosed in U.S. Pat. No. 3,484,237 issued Dec. 16, 1969. The conductive substrate can comprise an aluminum layer sprayed onto an insulating surface. The uniform emission effected by the negatively biased corona discharge electrode ll facilitates the placement of the electrode 11 much closer to the plate 18 than that heretofore achievable thereby greatly reducing the potential of supply 17 and further lowering the emitted current over that required by prior devices.

A three wire corotron discharge system is depicted in FIG. 2b adjacent an electrophotographic plate 18. The utilization of the layered discharge electrodes 11 of the present invention also greatly reduces the power requirements of this device. In a similar manner, the power requirements of the discharge electrodes 11 of the scorona discharge system of FIG. 20, the scorotron discharge system of FIG. 2d and the bird cage discharge system of FIG. 2e are greatly reduced when the inorganic dielectric layer of the present invention is incorporated over the metallic core of the electrode. Further, the currents associated with the grids 25 are correspondingly reduced.

The mechanism by which such uniform emission is obtained from the corona electrode of the present invention has not been elucidated. However, several theories may be conjectured.

. Firstly, it is noted that a negatively biased metallic filament emits a corona discharge from nodal points which may be readily observed. The non-uniform emission is attributed to work function variations in the surface layer of the metal which may be occasioned by non-uniform oxide formations, grain boundary variations and/or adsorbed gases, and to non-uniform electric field variations caused by surfaceasparities. Once electrons are emitted from a localized area of the metallic surface, that area is bombarded with positive ions thereby creating secondary emission at the localized sites.

This nodal emission phenomena can be visually observed with any metallic corona filament electrode which is negatively biased including those of the most noble metals such as gold. I

When such a metal filament has a dielectric inorganic material layer such as a metal oxide or ceramic material bonded thereto, the distinct emission sites are no longer readily observable.

Thus, the addition of a dielectric material layer may result in the obtainment of a more uniform work function at the surface of the corona discharge electrode. The uniformity of the emission may be attributed to the very high secondary electron emission yield of the dielectric material.

Further, the uniform corona discharge could be attributed to a resistive shunting effect created by the dielectric layer. That is, a voltage drop is known to exist across the dielectric material as well as across the boundary between the corona emission electrode and the requisite adjacent conductive surface such as the surface 19 of FIG. 2a. Assuming that the dielectric material has substantially uniform resistivity, those surface locations emitting electrons create a higher voltage drop across the dielectric layer due to the relatively high current thereto. The higher voltage drop across the material at a localized point limits the current emitted from that point. Therefore, many such emission sites are necessary to attain the total current equivalent to that obtained with widely spaced highly emitting nodes of a metallic filament. This effect may thus be thought of as a current limiting effect or a resistive shunting effect.

The mechanism of uniform emission can also be explained by an enhanced field effect theory. That is, a high relatively uniform electric field is created across the dielectric layer by the buildup of positive ions on the outer surface of the dielectric material. Electrons are then injected from the metal of the core material into the dielectric layer. The injected electrons then tunnel across the dielectric layer and/or avalanches are precipated in the dielectric layer (Malter effect). The electrons then emit from the surface of the dielectric layer.

While the mechanism by which the inorganic dielectric layer promotes uniform emission from a negative biased metallic electrode is not fully understood, it is noted that various organic dielectric materials have been utilized solely or in combination with inorganic dielectric materials as a coating for the metallic electrode. None of the discharge electrodes coated with such organic materials has exhibited more than a minimal improvement in emission uniformity and most of the organic coatings rapidly break down due to material failures. It is presumed that these organic materials are chemically unstable in a negative corona environment and/or they will not support a sufficiently high electric field thereby preventing their use in obtaining uniform negative corona emission.

The following are examples of the present invention in detail. The examples are included merely to aid in the understanding of the invention, and variations may be made by one skilled in the art without departing from the spirit and scope of this invention.

EXAMPLE I A coating of silicon nitride was deposited by chemical vapor deposition onto the surface of a 0.0025 inch diameter tungsten wire. This coating was examined by scanning electron microscopy and found to be uniform in topography at a thickness of approximately 500A. The silicon nitride coated wire was then assembled in a scorona charging device along with a control 0.0025 inch diameter tungsten wire coated with a 100 microinch coating of gold.

A voltage of approximately 7,000 volts was then impressed on each wire at a current level of approximately 50 microamperes per inch. Each wire emits a corona discharge. When examined visually, the gold coated tungsten wire has an emission pattern of moving beads spaced between one-sixteenth to one-eighth inch apart. The silicon nitride coated wire initially appears to have a non-uniform emission, but within ten minutes of turn-on, the emission becomes a uniform glow along the length of the wire with no readily observable emission nodes.

After uniform emission is obtained on the silicon nitride coated tungsten wire, no degradation is thereafter noted when the potential impressed on the corona electrode is removed and thereafter turned-0n again.

Current density scans of the emission from the. wires reveal that the silicon nitride coated wire has a substantially more uniform emission pattern than the gold coated wire, there being a 7:1 improvement in the relative peak to peak values. Further, the peak to peak value of the current density scan of the silicon nitride wire is approximately the same as that previously recorded by scanning a gold coated wire which was positively biased to emit at approximately the same current density as the negative case.

The silicon nitride coated wire and the gold coated wire were then allowed to continuely emit with the emission patterns being periodically checked both visually and by current density scans. After 1,000 hours of operation, the silicon nitride coated wire was still emitting uniformly and the gold coated wire had degraded to emitting from widely spaced stationary nodes.

The silicon nitride coated wire was then removed from the test apparatus and placed with two other similar silicon nitride coated wires in a three wire scorona charging system and incorporated as the charge corona of an IBM Copier ll office copying machine. A potentialof approximately 7,000 volts with a current of approximately 50 microamperes per inch (a total system electrode current of L5 milliamperes) was applied to the wires. Thus, the typical operating point of the corona electrodes was 10.5 volt'amperes. The wires were located approximately 0.3 inches from the scorona grid which was located approximately 0.05 inches from the surface of the moving photoconductor drum of the machine. Continuous toned copies from the copying machine showed no evidence of uneven charging. This test was repeated at 83 F. and at a relative humidity (R.H.)

of percent without noticeable degradation of copy quality.

It is noted that each hour of. corona on time is approximately equal to 1,500 copies produced by an IBM Copier II running continuously. Thus, the silicon nitride coated wire having an on time duration in excess of 1,000 hours is equivalent to a corona in a machine evironment being on for a period of time to produce 1,500,000 copies.

The charge corona of an IBM Copier ll copying machine comprises a bird cage charging system utilizing three gold plated tungsten wires located 0.5 inches from the grid wires which are located approximately 0.05 inches from the photoconductor. A -14,000 volt supply supplies approximately 80 microamperes per inch of current to the electrodes. The typical operating point is at l2.5 K volts or 31.25 voltamperes. No uneven charging is noted. Therefore, power consumption is approximately reduced by a factor of three by utilizing the dielectric coated wires in a scorona configuration. A gold coated tungsten wire cannot be utilized in a scorona device because of noding unless high potentials and current densities are utilized with separation.

EXAMPLE ll A coating of silicon was deposited by chemical vapor deposition from SiH (Silane)' onto the surfaces of several 0.0025 inch diameter tungsten wires. The thickness of the coatings so deposited varied from wire to wire from 0.0001 inch to 0.0003 inches. The wires were then assembled in the scorona charging device of Example I and a potential of approximately 7,000 volts was impressed on the wires at a current level of approximately 50 microamperes per inch. Initial emission from the wires was very non-uniform. After a burn-in time varying from 5 minutes to 2 hours, uniform emission was achieved. Continuous improvement in uniformity of emission occurred during the burn-in time. Once uniform emission was achieved, the wires continued to uniformly emit regardless of interruptions of the applied voltage. Uniform emission continued in excess of 700 hours of continuous operation.

When subjected to environmental extremes, it was noted that the emission became nodal at high temperatures and high relative humidities (83 F 80% RH. and 75 F 60percent R.H.). The nodal emission at such high humidities is attributed to the adsorption of water by the silicon oxide surface. Such water adsorption causes the resistivity of the silicon oxide surface to markedly decrease creating low resistance paths to .nodal emission sites.

EXAMPLE III which time the surface degraded due to wire vibration causing the brittle coating to crack and spall.

' 7 EXAMPLE iv A phosphosilicate glass coating P., si., ,,,o, was applied to several tungsten wires 0.0025 inches in diameter. The coatings were uniform and varied in thickness from 2,000 to 6,000A and were noted to be quite ductile. The wires were then placed in a corotron charging unit a potential of approximately -7,000 volts was applied thereto at a current level of microamperes per inch. The emission from the wires was noted to be uniform with no initial burn-in required. The wires have continued to emit uniformly in excess of 600 hours.

The wires operate to provide uniform emission throughout a wide range of environments including highhumidity and temperature.

EXAMPLE V glow along the length of the wire. This wire is then placed in the scorona charging unit and placed in the IBM Copier ll copying machine as described in Example 1. Continuous toned copies produced by the machine show no evidence of non-uniform charging after testing up to 83 F R.H. A similar wire is then placed in a corotron charging device and allowed to emit continuously. Uniform emission is noted for a period of 18 hours at which point the wire failed by necking down and fracture.

EXAMPLE VI An aluminum ribbon of 99.84percent aluminum was assembled in a scorona charging device. The ribbon was [/16 inch by 0.0025 inch. An electrical potential of approximately -8,000 volts at a current level of approximately 50 microamperes per inch is applied to the ribbon. The initial corona emission is observed to be nodal; however, after approximately 20 minutes of emission, the emission becomes a uniform glow along the top of the ribbon. The surface uniformly emitted for a period of 5 hours after which nodal emission occurred.

EXAMPLE VI] A 0.0025 inch diameter gold coated tungsten wire was dip coated .with aluminum by drawing the wire through a molten bath of 1100 aluminum. The resultant aluminum coating was approximately 0.0005 inches in thickness with uniform coverage of the wire surface. The coated wire was placed in a scorona charging device and potential of approximately -7,000 volts at a current level of approximately 50 microamperes per inch was impressed thereon. The wire initially emitted non-uniformly, but after approximately 30 minutes, uniform emission was achieved. The wire was allowed to emit continuously for a period of approximately 8 hours at which time nodal emission appeared. Examination of the wire surface revealed that the aluminum layer had completely oxidized and that failure occurred at points were the aluminum oxide had cracked or spalled from the wire surface.

EXAMPLE VI]! 'at about 142 hours, the emission from the corona occurring from stationary nodal points at that time.

EXAMPLE lX A coating of magnesium was deposited by ion plating magnesium onto the surface of a 0.0025 inch diameter tungsten wire. The coating was approximately 500A. thick. The wire was placed in a corotron charging device and a potential of approximately 7,000 volts at a current level of approximately 50 microamperes was applied to the wire. Non-uniform emission was noted for approximately 5 minutes. Thereafter, the wire emitted uniformly in excess of l00 hours of operation.

The following examples describe various organic dielectric materials which were coated onto a metallic wire in an attempt to achieve uniform negative corona emission.

EXAMPLE X A coating of a silicone rubber (RTV-60 supplied by the General Electric Company) filled with 47percent by weight ferric oxide particles (Fe O and silica (SiO was applied to a 0.0025 inch diameter tungsten wire. The coating was approximately 0.0001 inch thick and uniformly covered the wire surface. When this wire was placed in the scorona charging device of Example I and a negative potential of 7,000 volts at a current level of 50 microamperes per inch was applied, the emission pattern was nodal and no improvement occurred after an emission period of 1 hour. Examination thereafter of the wire surface revealed the coating to have been partially removed during the 1 hour emission cycle.

EXAMPLE XI Submicron alumina powder was mixed with a silicone resin (SR420 supplied by the General Electric Company). This mixture was subsequently applied to a gold coated tungsten wire by dip coating to achieve a final coating thickness of approximately 0.000] inches. The coated wire was placed in the scorona charging device of Example I and a negative potential of approximately 7,000 volts at a current level of 50 microamperes per inch was impressed thereon. The emission was nodal at the initial turn-on and remained poor for a period of approximately 1 hour. At this time the wire was removed and examination revealed that the coating had been completely removed.

EXAMPLE XII A coating of 85percent by volume solution grade polyurathane (Estane 5740 produced by B. F. Goodrich) mixed with percent by volume graphite pigment was dip coated onto a 0.0025 inch diameter gold coated tungsten wire. The coating was approximately 0.0002 inches thick and uniformly covered the wire surface. The wire was placed in the scorona charging device of Example I and a potential of -7,000 volts at a current level of 50 microamperes per inch was applied thereto. Nodal emission was observed for approximately the first hour of operation. The nodes were more closely spaced than those produced by the wires of Examples X and X], but were readily observable. During the second hour of operation, extremely nonuniform emission occurred.

EXAMPLE XIII A composition of an acrilic resin and a volatile carrier (Krylon supplied by Borden, lnc.) was spray coated onto a 0.0025 inch diameter gold coated tungsten wire. The coating was approximately 0.0001 inch thick. Aluminum oxide was dusted onto the surface of the coating material. The wire was thereafter placed in the scorona charging device of Example I and a -7,000 volt potential at a current level of 50 microamperes per inch was applied thereto. Nodal emission was noted for approximately one hour. The wire was then observed and the coating was found to be removed.

From the foregoing, it is readily observed that a corona electrode comprising a conductive substrate coated with an inorganic dielectric material such as a ceramic material or a metal oxide provides uniform electron emission when negatively biased to a corona discharge potential. Further, this phenomenon is observed only when inorganic materials are utilized, the utilization of organic materials failing to produce the desired uniform corona emission. It is further noted that although some of the inorganic materials require a burn-in time (attributable to the formation of oxides, etc.), all of such materials exhibit uniform emission for a considerable time duration. It is further noted that some of the dielectric inorganic materials exhibit uniform emission over a wide range of temperatures and humidities to which a device such as an office copying machine could be subjected.

While the invention has been particularly shown and described with reference to preferred embodiments thereof, it will be understood by those skilled in the art that the foregoing and other changes in form and details may be made therein without departing from the spirit and scope of the invention.

What is claimed is:

l. A negative corona discharge system including:

a corona discharge electrode comprising a core having at least an outer conductive layer and a thin layer of a metal oxide dielectric material intimately bonded to the outer periphery of said conductive layer;

a negative direct current potential source connected to said corona discharge electrode for generating a uniform negative corona discharge from said corona discharge electrode.

2. The negative-corona discharge system set forth in claim 1 wherein said core consists of conductive metallic material. I

3. The negative corona discharge system set forth in claim 1 wherein said outer conductive layer consists of a metallic material and wherein said metal oxide is an oxide of the metallic material of said outer conductive layer.

4. A negative corona discharge system according to claim 3 wherein said core consists of a conductive metallic material.

5. The negative corona discharge system set forth in claim 3 wherein said core comprises aluminum and said metal oxide comprises aluminum oxide.

6. The negative corona discharge system set forth in claim 3 wherein said core comprises zinc and said metal oxide comprises zinc oxide.

7. The negative corona discharge system set forth in claim 3 wherein said core comprises magnesium and said metal oxide comprises magnesium oxide.

8. A negative corona discharge system including:

a corona discharge electrode comprising a core having at least an outer conductive layer and a thin layer of a ceramic dielectric material intimately bonded to the outer periphery of said conductive layer;

a negative direct current potential source connected to said corona discharge electrode for generating a uniform negative corona discharge from said corona discharge electrode.

9. The negative corona discharge system set forth in claim 8 wherein said ceramic material comprises silicon nitride.

10. Thenegative corona discharge system set forth in claim 8 wherein said ceramic material comprises silicon dioxide.

11. The negative corona discharge system set forth in claim 8 wherein said ceramic material comprises a phosphosilicate glass.

12. An improved corona discharge electrode for imposing a uniform negative electrostatic charge on an electrophotographic plate comprising an insulating layer mounted on a conductive backing member comprising:

at least one corona discharge electrode adjacent to and spaced from said insulating layer, said corona discharge electrode comprising a core having at least an outer layer of metal and a thin layer of a metal oxide dielectric material intimately bonded to said outer metal layer;

a negative direct current potential source connected to said corona discharge electrode to provide a negative potential between said corona discharge electrode and said conductive backing member for generating a negative corona discharge from said corona discharge electrode thereby imposing a uniform electrostatic charge on said insulating layer of said electrophotographic plate.

13. The improved corona discharge electrode of claim 12 wherein said metal oxide dielectric material is a metal oxide of the metal of said outer layer.

14. An improved corona discharge electrode for imposing a uniform negative electrostatic charge on an electrophotographic plate comprising an insulating layer mounted on a conductive backing member comprising:

at least one corona discharge electrode adjacent to and spaced from said insulating layer, said corona discharge electrode comprising a core having at least an outer layer of metal and a thin ayer of a ceramic dielectric material intimately bonded to said outer metal layer;

a negative direct current potential source connected to said corona discharge electrode to provide a negative potential between said corona discharge electrode and said conductive backing member for generating a negative corona discharge from said corona discharge electrode thereby improsing a uniform electrostatic charge on said insulating layer of said electrophotographic plate. 

1. A negative corona discharge system including: a corona discharge electrode comprising a core having at least an outer conductive layer and a thin layer of a metal oxide dielectric material intimately bonded to the outer periphery of said conductive layer; a negative direct current potential source connected to said corona discharge electrode for generating a uniform negative corona discharge from said corona discharge electrode.
 2. The negative corona discharge system set forth in claim 1 wherein said core consists of conductive metallic material.
 3. The negative corona discharge system set forth in claim 1 wherein said outer conductive layer consists of a metallic material and wherein said metal oxide is an oxide of the metallic material of said outer conductive layer.
 4. A negative corona discharge system according to claim 3 wherein said core consists of a conductive metallic material.
 5. The negative corona discharge system set forth in claim 3 wherein said core comprises aluminum and said metal oxide comprises aluminum oxide.
 6. The negative corona discharge system set forth in claim 3 wherein said core comprises zinc and said metal oxide comprises zinc oxide.
 7. The negative corona discharge system set forth in claim 3 wherein said core comprises magnesium and said metal oxide comprises magnesium oxide.
 8. A negative corona discharge system including: a corona discharge electrode comprising a core having at least an outer conductive layer and a thin layer of a ceramic dielectric material intimately bonded to the outer periphery of said conductive layer; a negative direct current potential source connected to said corona discharge electrode for generating a uniform negative corona discharge from said corona discharge electrode.
 9. The negative corona discharge system set forth in claim 8 wherein said ceramic material comprises silicon nitride.
 10. The negative corona discharge system set forth in claim 8 wherein said ceramic material comprises silicon dioxide.
 11. The negative corona discharge system set forth in claim 8 wherein said ceramic material comprises a phosphosilicate glass.
 12. An improved corona discharge electrode for imposing a uniform negative electrostatic charge on an electrophotographic plate comprising an insulating layer mounted on a conductive backing member comprising: at least one corona discharge electrode adjacent to and spaced from said insulating layer, said corona discharge electrode comprising a core having at least an outer layer of metal and a thin layer of a metal oxide dielectric material intimately bonded to said outer metal layer; a negative direct current potential source connected to said corona discharge electrode to provide a negative potential between said corona discharge electrode and said conductive backing member for generating a negative corona discharge from said corona discharge electrode thereby imposing a uniform electrostatic charge on said insulating Layer of said electrophotographic plate.
 13. The improved corona discharge electrode of claim 12 wherein said metal oxide dielectric material is a metal oxide of the metal of said outer layer.
 14. An improved corona discharge electrode for imposing a uniform negative electrostatic charge on an electrophotographic plate comprising an insulating layer mounted on a conductive backing member comprising: at least one corona discharge electrode adjacent to and spaced from said insulating layer, said corona discharge electrode comprising a core having at least an outer layer of metal and a thin ayer of a ceramic dielectric material intimately bonded to said outer metal layer; a negative direct current potential source connected to said corona discharge electrode to provide a negative potential between said corona discharge electrode and said conductive backing member for generating a negative corona discharge from said corona discharge electrode thereby improsing a uniform electrostatic charge on said insulating layer of said electrophotographic plate. 